Mass-Properties Measurements For Large Structures May Be Safer, More Accurate, Efficient And Less Costly

Three Dryden researchers are studying the Dynamic Inertia Measurement Method, or DIMM. DIMM could augment ground-vibration tests with additional software and hardware to make mass properties measurements for large structures more accurate and efficient, less costly and safer.

Claudia Herrera and Leonard Voelker were awarded a NASA Innovation Fund Program grant last summer to use the DIMM to calculate mass properties on large structures. It had previously been proven that the DIMM could be used effectively to measure mass properties on desktop-sized objects. The latest research was completed last fall in the Dryden Flight Loads Laboratory.

"Conventional methods for measuring mass properties are an incredibly big production," said Herrera, principal investigator on the project.

"It takes a lot of people and it exposes the vehicle to a lot of risk because for just about every setup you have to do a critical lift. You usually have to design some big, expensive test fixtures and then you have to analyze and test those fixtures to show they are structurally sound for support of the test article.

"In addition, for just about every mass property value you are trying to measure, you need a different configuration, which adds risk to the test article."

Not so with the DIMM, she said. Because most of what is required for its use is available during ground-vibration testing, mass properties measurements could be taken with just a little more effort.

As an example, it took a month to obtain information on seven of 10 mass properties measurements required to construct a full mass properties matrix on the crew module used as part of the May 6 Orion Pad Abort 1 test at White Sands Missile Range, N.M., Herrera said. With the DIMM, all the mass properties could have been obtained in about one quarter of the time "with one configuration and just one critical lift."

Because of the ease of use, Herrera said using the DIMM for large objects could make it valuable for a number of different design projects, especially in the aerospace and automotive worlds.

"People don't complete a lot of mass properties measurements because of the complexity. However, with this method, people would look at the magnitude of difference in obtaining that information, compared to traditional methods, and use it," she said.

Traditional methods and the DIMM concept were compared with separate evaluations of a 17,000-pound test article built for the research. DIMM test methods were evaluated with sensors, including six-degree-of-freedom load cells to measure the support force, or the force on the structure that is supporting the test article. The excitation, or force input to the structure to make it vibrate, was achieved through use of an impact hammer and a shaker and measured through use of a tri-axial force transducer. Seismic accelerometers were used to measure the response from the structure as a result of the excitation.

Hardware issues during the tests limited the analysis window, Herrera said. For that reason, further study is needed for DIMM application on large test articles for the full-analysis window, which can reduce uncertainty in results before moving on to the next step: proving and demonstrating the method on a full-size aircraft.

The research showed promise and additional investigation could validate use of the DIMM, Herrera said. Success could lead to a day when the DIMM is used for measuring mass properties that are more accurate, less expensive and time intensive and with less risk.